How strong is the evidence - based on macroinvertebrate community responses - that river restoration works?

We reviewed river rehabilitation studies published from 1984 to 2019 to identify factors that might limit effective rehabilitation. This encompasses 89 papers that reported outcomes of 379 independent projects. We found that methods used to evaluate the outcomes of rehabilitation projects may have failed to properly assess the outcomes, which has led to a poor diagnosis of both the "problem" and the effectiveness of any "solution". We identified four methodological limitations that have often precluded the rigorous assessment of the effectiveness of stream rehabilitation: (1) The most comprehensive Before-After-Control-Impact (BACI) study design was not common practice. (2) Most studies sampled rivers for only one season following rehabilitation, and therefore could not account for seasonal or annual variations that could affect macroinvertebrate community composition. (3) Multi-habitat sampling - to comprehensively represent macroinvertebrate communities in study reaches - was rarely applied. (4) The most commonly employed indicators of rehabilitation success were macroinvertebrate taxa richness and diversity, even though these measures may fail to identify other consequential changes in ecosystem structure and function. Ecosystem functional indicators such as macroinvertebrate Functional Feeding Group (FFG) and Ephemeroptera-Plecoptera-Trichoptera (EPT) diversity, density, biomass and secondary production often had better responses, but were rarely assessed. Future rehabilitation projects and monitoring of their outcomes should aim to rehabilitate ecosystem functions, not solely structures. BACI monitoring design and multi-habitat sampling at in-stream biotope level are required to detect physical and biological changes that may otherwise go unnoticed. The presence of upstream population sources can facilitate biotic recolonisation and decrease the post-project time frame of recovery. (C) 2019 European Regional Centre for Ecohydrology of the Polish Academy of Sciences. Published by Elsevier B.V. All rights reserved.